Ink jet printing systems can be divided into the continuous jet type and the drop-on-demand type systems. In the former, a succession of drops are ejected from a small nozzle and projected electrostatically toward a recording medium such as a paper sheet. Selected drops are deflected electrostatically into a gutter, while the remaining drops impinge on and adhere to the sheet to form a character or figure on the paper according to a predetermined dot matrix. In the drop-on-demand or impulse jet type printer, the volume of a pressure chamber filled with ink is suddenly decreased by the impression of an electrical driving pulse and an ink drop is jetted from a nozzle communicating with the chamber. Thus, a single drop of ink is transferred to the paper by a single driving pulse, following which the system returns to its original state. In use, a succession of such droplets is ejected in response to a succession of drive pulses to form a character or figure on the paper according to a predetermined dot matrix. Examples of such printers are disclosed in U.S. Pat. Nos. 3,653,932; 3,715,219 and in U.S. application Ser. No. 571,094, filed Jan. 16, 1984, entitled Droplet Ejector, owned by the assignee of the present application, now abandoned. While the present invention is applicable to both types of ink jet printing, we will describe it specifically in relation to drop-on-demand ink jet printing.
There are several requirements for an ink composition used in an impulse jet printing process. The physical properties of the ink such as viscosity and surface tension must be within proper ranges if the ink is to be jetted properly. The ink must not clog the nozzle orifice. It must produce images of sufficient optical density. Further, the ink must have a high rate of fixing to the recording medium and not wrinkle, curl or otherwise adversely affect the medium. Further, the ink must be able to form high resolution substantially circular dots on the recording medium which are resistant to water, common solvents, light and abrasion. Since we are concerned here specifically with ink jet color printing, it is desirable if not essential also that the ink be capable of printing in three primary colors as well as in black to produce characters and pictures in a very wide variety of different colors and hues. Satisfactory color printing by ink jet involves, then, the formation on the recording medium of a multiplicity of colored dots or spots of differing color intensities, depending upon the requirements of the various parts of the character or picture being printed on the medium.
This wide color printing spectrum can be achieved using three or four different color inks either by an additive color mixing process or a subtractive process. In the former, red, green, blue and sometimes black ink drops are deposited on the medium side by side in a dot matrix. The different colored drops are integrated in the observer's eyes so that he perceives a color dependent upon the relative numbers and/or sizes of the different color dots at each part of the printed character or picture. In the subtractive process, the printer deposits ink drops of the primary subtractive colors, namely cyan, magenta and yellow, as well as black, on the medium in superposition in a dot matrix so that each dot is composed of one or more layers, up to three or four, of printing inks having different color intensities, depending upon the color requirements of the particular part of the character or picture. Each dot layer absorbs a portion of the spectrum of the ambient light illuminating the medium so that the viewer's eye senses the remainder of the light spectrum. Thus, when several ink layers each of a different primary color are present in a single dot in the matrix, several spectral portions are absorbed simultaneously so that a smaller part of the spectrum of the incident light is reflected causing the viewer's eye to sense a mixed color produced by the subtractive mixture of the primary colors.
Thus, true color printing using a subtractive color mixing process depends upon the precision of the printing process as well as the optical properties of the different color printing inks. The color graphics industry prefers to print by subtractive color mixing because the resultant color printing is brighter and more vivid than that resulting from additive color mixing, particularly when the power of the ambient light is relatively low.
Presently, two different types of inks are usually used for ink jet printing, namely water-based inks and oil-base inks and there are many known water- and oil-based inks that can be jet-printed onto a printing medium to produce characters and pictures in color by both the additive and the preferred subtractive color mixing processes. However, such inks have certain drawbacks. More particularly, water-based inks exist on the medium, usually paper, as very thin layers and they get imbibed into the paper. The resultant interaction of the water and the paper fibers distorts the paper. It also results in the incident light being reflected diffusely from the color dots so that the observed printed colors are muted. Of course, the paper can be specially treated to avoid this problem. However, that is a limiting factor and adds to the expense of the color printing process. Oil-based inks also tend to penetrate a paper printing medium producing diffuse reflections which mute the observed colors. Further, they have low surface tensions so that, when deposited on the paper, they tend to wick along the paper fibers resulting in overly large and rather irregular printing dots.
According it would be desirable to be able to provide subtractive color printing by an ink jetting process which is brighter and more vivid than that produced by conventional ink jet color printing systems. This necessarily involves the providing of an ink composition which does not adversely affect the printing medium upon which it is placed and whose colors are not muted or otherwise adversely affected by that printing medium. Preferably also the ink composition should be applicable to ordinary untreated paper which is the most common and least expensive printing medium.